1. Field of the Invention
The present invention relates generally to methods for separating plasma or serum from whole blood. More particularly, the invention relates to devices capable of separating plasma or serum from whole blood comprising a hydrophilic sintered porous material in which at least one red blood cell agglutinating agent has been incorporated. Agglutinated blood cells are removed from whole blood by the sieving action of the matrix of the sintered porous material, and optional additional filter means.
The present invention also relates to devices and methods for collecting a predetermined amount of a plasma or serum sample for analysis in a diagnostic assay. More particularly, the invention relates to a matrix of sintered porous material, wherein the matrix provides a reproducible fluid uptake capacity.
2. Background
Modern clinical diagnostic methods are routinely carried out on blood samples. Unfortunately, red blood cells present in whole blood scatter and absorb light thus interfering with assay methodologies which measure either reflected or transmitted light. Other cells may interfere with particular determinations; for example, cholesterol determinations can be affected by cholesterol present in cell membranes. For this reason, many assay methodologies are carried out on plasma or serum which must be separated from a whole blood sample.
Centrifugation is a well known method in the art by which plasma (before clotting) and serum (after clotting) is separated from whole blood. Stratifying whole blood by centrifugation, however, is time consuming and requires cumbersome laboratory equipment. The use of red blood cell agglutinating agents such as those disclosed in Van Oss, et al., Vox. Sang. , vol. 34, pp 351-361 (1978) can be helpful in carrying out centrifugation and other red blood cell separation techniques.
Dojki, et al., U.S. Pat. No. 4,464,254, issued Aug. 7, 1984, disclose a piston device capable of isolating serum from an already stratified blood sample. The device consists of a piston head connected to an open-ended sampling tube. The piston head is composed of a one-way valve under which is located a cavity containing a porous plastic filter body. Insertion of the piston head-sampling tube assembly into a test tube containing a stratified sample of blood allows serum to pass through the filter body and valve into the interior of the sampling tube. The volume and purity of the serum which can be separated from the whole blood is contingent upon the completeness of the stratification of the blood.
Vogel, et al., U.S Pat. No. 4,477,575, issued Oct. 16, 1984, disclose a device and a process using the device to separate serum from whole blood by causing whole blood to pass into and through a layer of glass fibers with diameters from 0.2 to 5 microns and with a density of 0.1 to 0.5 g/cm.sup.3. The volume of plasma or serum which can be separated from whole blood by this device is disclosed to be less than 50% of the absorption volume of the glass fiber layer.
Zuk, U.S. Pat. No. 4,594,327, issued June 10, 1986, discloses an analytical method wherein a whole blood sample is combined with a red blood cell binding agent and the mixture is then filtered through a solid bibulous element to which is bound at least one specific binding pair member so as to remove the agglutinated red blood cells. The patent discloses anti-red blood cell antibodies, polymeric amino acids, such as polylysine, and lectins, such as wheat germ agglutinin, as suitable red blood cell binding agents for causing the aggregation of red blood cells in whole blood.
Hillman, et al., U.S. Pat. No. 4,753,776, issued June 28, 1988, disclose a device and a process using the device to separate serum from whole blood using capillary action to pass whole blood through a glass microfiber filter. The patent discloses an alternative embodiment in which whole blood is passed through a filter to which red blood cell agglutinins have been attached. Rather than retaining the red blood cells, however, the filter disclosed merely retards their flow, eventually allowing their escape.
Trasch, et al., EPO Publication No. 133,895 published Mar. 13, 1985, disclose a red blood cell retaining substrate and a process using the substrate for retaining red blood cells on filters thus allowing the recovery of plasma from whole blood. The red blood cell retaining substrates of the invention are stated to induce coagulation, but not hemolysis, so that the coagulated corpuscular components can be removed on a filter, while the plasma passes through. The publication discloses alternative embodiments where the retaining substrate is incorporated into the filter or into a pre-filter layer. The publication states that absorptive, porous, liquid permeable carriers or filters, in the form of paper, fleece, gel or tissues, comprised of cellulose, wool, glass fiber, asbestos, synthetic fibers, polymers or mixtures of the same, can be used as the absorptive materials for the retaining zone.
Most portable technigues for the separation of serum or plasma are limited with respect to speed and serum yield efficiency. Blood separation devices utilizing glass fiber membranes, for example, tend to separate serum at a relatively slow speed and tend to retain significant quantities of serum or plasma in the interstices of the membrane. Accordingly, there exists a desire in the art for improved devices providing rapid and efficient methods for serum and plasma separation.
Another difficulty encountered in the testing of blood samples is that it is generally necessary to measure a precise test sample volume of plasma or serum for use in diagnostic assays. This need for precision is typically met by having a trained technician use a sophisticated pipetting apparatus or by the use of expensive automated instruments. There are also test strip devices which use membrane or paper matrices to collect a plasma sample and transport that sample to a reaction zone on the test strip. Test strip devices, however, typically provide only that sample volume capacity which is needed to transport sample by capillary action through the strip to the reaction zone, and therefore a low level of precision is reguired. In test strips devices, the plasma recipient member only collects that amount of sample necessary to fill the strip which in turn ends the migration of the sample through the strip because the drawing force which causes sample subject to analysis in a test strip device is limited to that amount which passes through a defined detection zone on the test strip before the strip is filled.